1. Field of the Invention
This invention relates generally to systems and methods for performing a radiation procedure, and more specifically, to systems and methods for obtaining images using a radiation machine.
2. Background of the Invention
Computed tomography is an imaging technique that has been widely used in the medical field. In a procedure for computed tomography, an x-ray source and a detector apparatus are positioned on opposite sides of a portion of a patient under examination. The x-ray source generates and directs a x-ray beam towards the patient, while the detector apparatus measures the x-ray absorption at a plurality of transmission paths defined by the x-ray beam during the process. The detector apparatus produces a voltage proportional to the intensity of incident x-rays, and the voltage is read and digitized for subsequent processing in a computer. By taking thousands of readings from multiple angles around the patient, relatively massive amounts of data are thus accumulated. The accumulated data are then analyzed and processed for reconstruction of a matrix (visual or otherwise), which constitutes a depiction of a density function of the bodily section being examined. By considering one or more of such sections, a skilled diagnostician can often diagnose various bodily ailments such as tumors, blood clots, etc.
When using computed tomography to examine bodily structures of a patient, a filter is generally placed between the patient and the x-ray source for modulating an intensity of an x-ray beam impinging on the patient during a CT scanning. Such filter is designed to reduce a dose to the patient modestly (reduce skin exposure by 30-40%) while having no detrimental effect on image quality. The cross-sectional shape of the filter, which is thin in the middle and thicker at its edges, is so configured such that the filter attenuates the beam more where the patient is the thinnest. Such arrangement compensates for differences in thicknesses over the cross-section of the patient's body so that an intensity of the x-ray beam exiting the patient is approximately uniform. Because existing filters are specially configured to provide an uniform image quality for an entire image of the patient's body, existing filters do not preferentially increase a dose (and hence improve an image quality) to a target region within the patient's body. In some cases, it may be desirable to obtain an image having a non-uniform image quality (e.g., it may be desirable to obtain good quality image for only a target region).
Also, there is a need to further reduce an overall dose of radiation delivered to a patient during a CT scan. Recently, some researchers have suggested the use of local tomography to reduce radiation dosage delivered to a patient. In this approach, the fan angle of the beam is reduced so that only a part of the patient is exposed to radiation during a CT scan. Although such technique substantially reduces radiation exposure to the patient, it is susceptible to substantial artifacts if highly attenuating structures are outside the region of interest. There are special local tomography reconstruction algorithms that can be used for reconstruction of images. However, these algorithms do not provide accurate CT numbers (Hounsfield Units). For the foregoing reason, local tomography is not suited to the use of CT scans for treatment planning applications, which requires an accurate determination of a density function of the bodily structure being examined.
For the foregoing, it would be desirable to have a method and a system for filtering radiation beam such that a dose to a target region can be increased to improve an image quality of the target region. It would also be desirable to have a method and a system for filtering radiation beam such that an overall dose of radiation delivered to a patient can be minimized or at least reduced during a radiation procedure.